380 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 315. 



eteam-boiler, properly arranged and insulated, therefore constitutes a most 

 powerful electrical machine ; and by mean3 of an apparatus of this character, 

 constructed some time since in London, flashes of electricity were caused to 

 emanate from the prime conductor more than 22 inches in length. 



749. The Insulating Stool, which is a usual 

 Bulating Stool ? appendage to an electrical machine, consists of 

 a board of hard-baked wood, supported on 

 glass legs covered with varnish. (See Fig. 315.) It is useful for 

 insulating any body charged with electricity; and a person 

 standing upon such a stool, and in communication with a 

 prime conductor, will become charged with electricity. 



n 



Fig. 316. 



-<c 



Dischargmg Rods are brass 



What are Dis- a ^ ° 5 •*t- i n 



charging Rods f ^ods termmatmg with balls, or 



with points, fi.xed to glass handles. 

 With these rods electricity may be taken from a 

 conductor without allowing the electrical charge 

 to pass through the body of the operator. Their 

 construction is represented in Fig. 316. 



„ An instrument called the " Unive'sai 



Fig 317 



Discharger," used to convey strong 



charges of electricity through various 

 substances, is represented by Fig. 317. 

 It consists of two glass standards, 

 through the top of which two metalUc 

 wires slide freely; these wires are 

 pointed at the end, t, but have balls 

 screwed upon them ; the other ends are furnished with rings. The balls rest 

 on a table of boxwood, into which a slip of ivory, or thick glass, is inlaid. 

 Sometimes a press, p', is substituted for the table, between which any sub- 

 stance necessary to be pressed, during the discharge, is held firm. 



750. An Electrophorus is a simple appara- 

 tus, in which a small charge of electricity may- 

 be generated by induction ; and this, communicated suc- 

 cessively to an insulated conductor, may produce a charge 

 of indefinite amount. 



It consists of a circular cake of resin (shell-lac), r. Fig. 318, 

 laid upon a metallic plate ; upon this cake, the surface of which 

 has been negatively electrified by rubbing it with dry silk or fur, 

 is placed a metallic cover, M, somewhat smaller in diam- 

 eter, and furnished with a glass insulating handle, h. 

 The negative electricity of the resin, by acting induc- 

 tively upon the two electricities combined in the cover, 

 separates them — the positive being attracted to the 

 under surface, and the negative repelled to the upper, 

 on touching the cover with the finger, all the negative 



What is an 

 Electrophjrus ? 



Describe the 

 action of the 

 electrophorus. 



Fig. 318. 



